This invention relates to electromedical apparatus and methods and particularly to electronic circuitry and methods for use in a cardiac defibrillator. More particularly, this invention relates to a method of detecting varying impedance in a cardiac cycle and delivering a defibrillating shock at a predetermined period in the cycle. The impedance timed defibrillation method of the invention is useful for determining the optimum time to deliver a defibrillating shock to a patient, particularly one having an implanted cardioverter/defibrillator, to derive maximum efficiency therefrom.
The implantable defibrillator is a well recognized and important tool for resuscitating cardiac arrest patients. Defibrillation of the human heart is accomplished by applying an electrical waveform to the cardiac muscle with appropriate electrodes, causing the cessation of rapid uncoordinated contractions of the heart (fibrillation) and restoration of normal beating of the heart.
In the past, various devices and methods have been used and proposed to defibrillate the human heart. However, these devices and methods have significant limitations and shortcomings, including the inability to deliver a defibrillation shock having maximum efficiency. Specifically, no device or method is known to base the delivery of a defibrillation shock on the impedance of the cardiac region of a human patient, despite the fact that the relationship of defibrillation shock efficiency to impedance is relatively well known. This invention is based, in part, on the recognition that the impedance of the pre-cordial region of a human patient constantly changes due to breathing and cardiac function. The term cardiac impedance as used herein refers to impedance due to such breathing and cardiac function as well as to other anatomical and physiological factors.
Despite the need in the art for a device and method which overcomes the limitations and problems of the prior art, none insofar as is known has been proposed or developed. Accordingly, it an object of the present invention to provide a device and method which overcome the limitations and shortcomings of the prior art. It is a further object of this invention to provide an improved defibrillation method and device which are reliable and effective at defibrillating a human heart, and which derive maximum efficiency from a defibrillation shock. A particular object of the present invention is to provide a method and device which deliver a defibrillation shock at a period in the cardiac cycle when cardiac impedance is at a low point.